Reduction of substituted benzenes

Relative Rates of Reduction of Substituted Benzenes Relative t<... 

Fig. 13. Dependence of half-wave potentials for the reduction of substituted benzenes on the total polar substituent constant, ax- Examples shown benzophenones [pH 0], benzophenone-oximes [pH 0], thiobenzophenones [pH 0], nitrobenzenes [pH 2-0], azo derivatives [pH 2-6]. (Taken from Zuman, 1969.)...

Rates of oxidation of isolated hydroxyl groups, 225 Rates of reduction, 14 Rates of reduction of substituted benzenes 14... 

Reduction of substituted benzenes with sodium (or lithium) in liquid ammonia in the presence of a proton source (such as methanol, ethanol, etc.) leads to a substituted, non-conjugated cyclohexadiene as a result of 1,4-addition of hydrogen (the Birch reduction).16 With benzene the product is cyclohexa-1,4-diene as a result of the following mechanistic pathway. 

Fig. 26. Relation of half-wave potentials for the reduction of substituted benzene derivatives in acid media to Hammett substituent constant trx. Benzophenones, benzophenone oximes and thiobenzophenones at pH 0 nitrobenzenes at pH 2,0 azo dyes at pH 2.6...

Several examples of the Birch reduction of substituted benzene derivatives are shown in the following equations. Note that substituents such as alkyl and alkoxy groups prefer to be attached to one of the carbons of the double bonds of the product, while a carboxyl group prefers to be attached to one of the singly bonded carbons. Benzene derivatives with other types of substituents are usually not employed as reactants in the Birch reduction because the substituents are not stable to the reaction conditions. 

The regiochemical course of the reduction of substituted benzenes is determined by the site of initial protonation of the radical anion species. Generally, electron-donating groups (D) retard electron transfer and remain on unsaturated carbons. 

FIGURE 13.69 In the Birch reduction of substituted benzenes, electronreleasing groups appear on one of the double bonds, but electron-withdrawing groups are located at the methylene position. 

In the reduction of substituted benzene rings, the anionic and radical centers are formed where one might expect—anions are stabilized by electron-withdrawing groups and destabilized by electron-donating groups. Examples of reduction of substituted benzenes are shown in Figure 13.41. 

Starting from a cisoid 1,3-diene, Fe2(CO)9 can yield, under mild thermal conditions, tricarbonyl(T -l,3-diene)iron complexes. Cyclohexa-1,4-dienes (readily available from the Birch reduction of substituted benzenes) often require prior conjugation, but conjugated 1,3-dienes (both cyclic and acylic) can be converted into tricarbonyliron complexes directly. Some of the earliest examples " of complexation of 1,3-dienes employed Fe2(CO)9 (eq 3, dates shown in parentheses). (Alternative reagents in the early days were Pentacarbonyliron or Dodecacarbonyltriiron.)... 

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